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Object Name: Messier 56
Alternative Designations: M56, NGC 6779
Object Type: Class X Globular Cluster
Right Ascension: 19 : 16.6 (h:m)
Declination: +30 : 11 (deg:m)
Distance: 32.9 (kly)
Visual Brightness: 8.3 (mag)
Apparent Dimension: 8.8 (arc min)
Locating Messier 56: Finding M56 isn’t too hard since it’s located about half-way between Beta Cygni (Albireo) and Gamma Lyrae. In both binoculars and finder scope, you will see a triangle of stars when progressing from Gamma towards the southeast that will almost point directly at it! Because M56 isn’t particularly large or bright, it does require dark skies – but makes a great object for both binoculars and small telescopes.
What You Are Looking At: Spanning about 85 light years in diameter, this incredible ball of stars is moving towards planet Earth at a speed of 145 kilometers per second… yet still remains about 32,900 light-years away. As one of the less dense of the Milky Way’s halo globulars, it is also less dense in variable stars – containing only perhaps a dozen. But out of that twelve, there a very special one… a Cepheid bright enough to be followed with amateur instruments. However, astronomers never stopped looking for the curious – and they found what they were looking for!
The CURiuos Variables Experiment (CURVE) was performed on M56 in 2008. “We surveyed a 6.5′×6.5′ field centered on the globular cluster M56 (NGC 6779) in a search for variable stars detecting seven variables, among which two objects are new identifications. One of the new variables is an RRLyrae star, the third star of that type in M56.” said P. Pietrukowicz (et al), “Comparison of the new observations and old photometric data for an RV Tauri variable V6 indicates a likely period change in the star. Its slow and negative rate of -0.005±0.003 d/yr would disagree with post-AGB evolution, however this could be a result of blue-loop evolution and/or random fluctuations of the period.”
But could other things exist inside M56? Events, perhaps, like nova? “Classical nova outbursts are the result of thermonuclear explosions on the surface of a white dwarf star in a close binary system. Material from the other star in the system (one not unlike our own sun) falls onto the surface of the white dwarf over thousands of years. The pressure at the base of this layer of accreted material builds up until thermonuclear reactions begin explosively. An Earth’s mass or more of material is ejected from the surface of the white dwarf at speeds of a few hundred to a few thousand kilometres per second. Old novae are therefore surrounded by shells of ejected matter illuminated by the light from the central binary system.” says Tim O’Brien, “We report the possible detection of diffuse X-ray emission in the environment of NGC 6779, and find the emission to be well aligned with the proper motion of the cluster. The position of the emission suggests we are observing heated ISM in the wake of the cluster that could be the result of an interaction between the intracluster medium and the halo gas surrounding it.”
History: When Charles Messier discovered M56 on January 23, 1779 he said: “Nebula without stars, having little light; M. Messier discovered it on the same day as he found the comet of 1779, January 19. On the 23rd, he determined its position by comparing it with the star 2 Cygni, according to Flamsteed: it is near the Milky Way; and close to it is a star of 10th magnitude. M. Messier reported it on the chart of the comet of 1779.”
However, it would be 1807 and Sir William Herschel to reveal its true nature. In his private notes he writes: “The 56th of the Connoiss. is a globular cluster of very compressed and very small stars. They are gradually more compressed towards the centre.” His son John would go on to observe it many times, even after cataloging it! His best description reads: “Large; round; very gradually brighter toward the middle. I see the stars which are very small and of different sizes. It fades gradually away to the borders.”
As always, it would be Admiral Smyth who would be perhaps a bit more descriptive when he included in his observing notes: “A globular cluster, in a splendid field, between the eastern joke of Lyra’s frame and the Swan’s head: it is 5 1/4 deg distant from Beta Lyrae, on the south-east line leading to Beta Cygni, which is about 3 1/2 deg further. This object was first registered by M. Messier in 1778, and, from his imperfect means, described as a nebula of feeble light, without a star. In 1784, it was resolved by Sir William Herschel, who, on gauging, considered its profundity to be of the 344th order.”
Enjoy this pincushion of stars!
Top M56 image credit, Palomar Observatory courtesy of Caltech, M56 courtesy of 2MASS, M56 Hubble Image and M56 color image courtesy NOAO/AURA/NSF.